Caption: Animal cell, cutaway illustration. The cell membrane surrounds internal structures (organelles) that include the nucleus (yellow-membraned, upper centre), which contains the nucleolus (light brown sphere, top centre). The nucleus contains DNA (deoxyribonucleic acid), the cell's genetic information. Surrounding the nucleus is endoplasmic reticulum (ER), a membrane bound organelle that is the site of lipid synthesis and the production of membrane-bound proteins. ER may be either rough (below nucleus), that is with ribosomes (small yellow dots), or smooth (right of nucleus), without ribosomes. The proteins are stored in the flattened membranes of the Golgi complex (bottom centre). Energy is generated by mitochondria (ovals at top left and bottom right). Lysosomes (top right of nucleus) and peroxisomes (green spheres) contain enzymes for digesting biomolecules. Vacuoles (bottom left) and vesicles (left centre) store and transport substances. Cells are the microscopic structural and functional units of all living organisms. For a labelled version of this image see C033/7309.